If you use joomla 1.5 and try to validate your DTD against W3C it is likely to have some errors related to ampersands.
This is due to a ¿bug? in default’s Joomla WYSIWYG editor (TinyMCE 2.0). By default this editor does not code the ampersands (&) to (&). If you try to write HTML using this editor and include a “&” you will notice it automatically changes “&” to “&”.
Getting rid of these annoying errors is quite simple. Just intall JCE (Joomla! Content Editor) plugin.
After the instalation is complete:
1. Log in to your administrator panel.
2. Click Site->Global Configuration.
3. Under the first tab (“Site”), find the parameter labelled “Default WYSIWYG Editor:”.
4. Select your JCE Editor 154 from the drop-down list (most likely the default editor was Tiny MCE 2.0.
5. Click the “Save” button in the toolbar.
Now validate your website again. You will get rid of those errors 
great! thank u so much!!